A DC voltage source is used as the power supply in many electronic devices. Generally, the DC voltage is derived from an AC power source. The AC voltage is rectified into an unregulated DC voltage by a rectifier bridge. The unregulated DC voltage is converted into a stable DC voltage as needed by a switching regulator.
A transformer or inductor is usually used as a tank element in the switching regulator. For example, a transformer is used in a flyback converter. A switch is electrically coupled to the primary winding of the transformer. The switch is turned ON and OFF so as to alternately store energy in the transformer and transfer the stored energy to the secondary winding of the transformer. An output capacitor is electrically coupled to the secondary winding of the transformer and a rectified voltage is generated thereon. The rectified voltage provides the DC output voltage of the switched power supply. The DC output voltage increases and decreases inversely with the load. The heavier the load, which means the higher the output current, the lower the output voltage, and vice versa. Generally, the DC output voltage is fed back to control compensation for the variation of the load.
There are two primary types of control methods used in the switching regulator. One is fixed frequency control and the other is variable frequency control. Although fixed frequency control is more widely used, it suffers from high switching loss and efficiency variation with load or input voltage due to the variable voltage across the switch.
An example of variable frequency control is quasi-resonant (QR) control. FIG. 1 shows an example waveform of a switching regulator circuit under QR control, wherein Vs is the voltage across the switch, CTRL is a control signal controlling the ON and OFF of the switch, and It is the current flowing through the tank element. In the example of FIG. 1, the switching regulator works under DCM (discontinuous current mode). After the current It flowing through the tank element goes to zero, the tank element becomes resonant with the parasitic capacitance of the switch. The switch is turned ON when the voltage Vs across the switch reaches its resonant valley so as to reduce switching loss. The switch is turned OFF when the current It flowing through the tank element becomes larger than a threshold level, which in the example of FIG. 1 may be a feedback signal related to the output voltage of the regulator.
Under QR control, the lighter the load, the shorter the ON time and OFF time of the switch. So under light load and high input voltage condition, the switching frequency may become too high and cause serious EMI (electromagnetic interference) problem. The EMI may not only reduce the quality of the power network, but also influence electrical devices connected to or placed near the switching power supply. Therefore, the switching frequency should be limited, for example, to be lower than 150 kHz.
Switching frequency may be limited by setting a minimum time limit, such as a minimum switching period or a minimum switch turn OFF time. In this approach, the switch can only be turned ON at the minimum voltage point after the minimum time limit, so as to limit the switching frequency while keeping the valley switching feature. However, this frequency limitation method may cause audible noise due to frequency hopping.
FIG. 2 is a waveform of a conventional QR controlled switching regulator with frequency limitation, wherein Tlimit is the minimum OFF time, and point A is a minimum voltage point across the switch. In practical application, the position of the minimum voltage point A may vary due to disturbance in the circuit. If the minimum voltage point A is slightly later, which means it occurs after the minimum OFF time Tlimit, the switch will be turned ON immediately. If the minimum voltage point A is slightly earlier, which means it occurs within the minimum OFF time Tlimit, the switch will be turned ON at the next minimum voltage point. From the description above, the OFF time may vary due to disturbance even when the load and the input voltage do not change. The variation of the OFF time will cause the switching frequency to hop in several switching periods, which may generate low frequency audible noise.